Apparatus for and method of establishing a density profile through the thickness of a panel

ABSTRACT

A measuring device (8) comprises an emitter (9) and a linear array (10) of detector elements (11 to 14). The emitter (9) emits a radiation beam (15) which enters into a narrow face (20) of a panel-like workpiece (7) at an angle (19). The individual rays (1 to 4) of the radiation beam (15) have different length paths (21 to 24) through the workpiece (7) and thus have differently attenuated output intensities. By difference calculations on the output intensities of the rays (1 to 4) thus obtained, one can calculate density values in the narrow face (20) and finally establish a density profile through the thickness (6) of the workpiece (7).

The invention relates to an apparatus for establishing a density profilethrough the thickness of a panel-like workpiece of non-homogeneousmaterial, for example a particle board panel or fibre board panel bondedwith glue or minerals, comprising: an emitter of a measuring device,whose radiation is arranged to pass through the workpiece, thereby beingattenuated by absorption in dependence upon the local density of theworkpiece, and is directed to a detector of the measuring device,wherein the detector produces an electrical signal corresponding to theoutput intensity of the attenuated radiation, and wherein the detectoris electrically connected to means for establishing the density profile.

The invention also relates to a method of establishing a density profilethrough the thickness of a panel-like workpiece of non-homogeneousmaterial, for example a particle board panel or fibre board panel,comprising the following steps:

A) directing radiation onto the workpiece from an emitter of a measuringdevice,

B) allowing the workpiece to be traversed by the radiation,

C) directing the radiation attenuated by absorption during the traverseaccording to step B) in dependence upon the local density of theworkpiece to a detector of the measuring device,

D) producing from the detector an electrical signal corresponding to theoutput intensity of the attenuated radiation, and

E) using the electrical signals obtained from step D) in an analyser toestablish the density profile.

In one known apparatus of this type (R Thompson et al: "Design andConstruction of a Profile Density Measurement System for the CompositeWood Products Industry" in: IEEE Proceedings--1989 Southeastcon, Session12D2, pages 1366 to 1371) a specimen cut from a particle board panel orfibre board panel is moved through a gamma radiation beam of a measuringdevice (see page 1367, FIG. 2). The gamma radiation travelsperpendicular to a narrow face of the specimen. One is talking hereabout a laboratory apparatus. The specimens are taken from the runningpanel production line and are analysed. It is disadvantageous howeverthat the panels have to be destroyed in order to remove the specimensand that there is a long period until the density profile is produced.The production process can therefore only be controlled with acorresponding delay.

An apparatus of the type first mentioned above is also known from C.Boehme: "The significance of the density profile for MDF" in: Holz alsRoh- und Werkstoff 50 (1992), pages 18 to 24. As shown in FIG. 10, avertical radiation beam is transmitted through a front shutter, thenthrough the length of a perpendicularly arranged test body of50×50×thickness mm³, then through a rear shutter and finally to adetector (scintillation counter). The test body is moved in steps of1/10 mm transversely to the direction of the radiation by means of astepping motor. The measuring period is 2 seconds for each step. Thedisadvantages mentioned above apply likewise to this apparatus.

From DE 34 29 135 C2 it is known per se to obtain a continuous thicknessmeasurement of rolled material, and in particular steel sheet of 1.5 to30 mm thickness, and among other things to display or print-out thetransverse profile of the rolled product. For this, the upper surface ofthe rolled product is irradiated from above by a fan-shaped beam from atleast one very strong gamma radiator 5, 6 in a measuring plane which isperpendicular to the direction of rolling. Detector beams 7, 8 with rowsof ionization chambers 18, 19 are arranged below the rolled product atright-angles to the direction of rolling. The fan-shaped form of thegamma radiation has the result that the radiation paths in the rolledproduct are longer the more the direction of the radiation departs fromthe vertical. Consequently, the analogue values of the measurementcurrents relating to the different measurement paths must be corrected.

It is the object of the invention to establish the density profile ofthe workpiece more rapidly.

This object is achieved by an apparatus having the features of claim 1.The workpieces can consist for example of particles or fibres of wood orfibres from one year old plants such as flax or bamboo. A mineral bindercan be used such as gypsum or cement for example. Gamma radiation orhighly penetrative X-ray radiation can be used in particular as suitableradiation for the measurement. The narrow faces of the workpiece are itstwo end faces and its two side faces. The apparatus according to theinvention can be used selectively with each of these narrow faces orwith any number of these narrow faces in succession or simultaneously,in order in the latter case to establish several density profiles fromone and the same workpiece, which makes possible a particularly completeand good analysis as to the quality of the workpiece. The workpieceitself can be either the known specimen cut from a panel, oralternatively the whole panel itself. In the latter case a completepanel is diverted from the manufacturing process and in anon-destructive manner can be used to establish one or more densityprofiles. The density profile can thus be established from the panelitself with particular advantage in a non-destructive testing mode andalso "on line", i e during the manufacturing process, at any of a numberof positions of one or more narrow faces of the panel. In all thesecases the density profile is established comparatively quickly andreliably. Thus, any tendency towards error in the manufacturing processcan be picked up early and corrected. This leads to a considerableimprovement in the quality of the workpieces and a reduction in thenumber of rejects.

The structure according to claim 2 is particularly simple from astructural and operational viewpoint.

The radiation beam according to claim 3 can have a thickness of 0.1 mmfor example. The width of the radiation beam is chosen so that with theemitter stationary the whole thickness of the workpiece is irradiatedsimultaneously.

According to claim 4, the emitter can be moved relative to thestationary workpiece and thus the relevant narrow face of the workpiececan be irradiated by the beam.

The detector elements according to claim 5 can be chosen in terms oftheir size and number according to the desired resolution. Detectorelements having a surface area of 1 μm² are realisable withoutdifficulty.

In the case of claim 6, the detector elements are preferably movedsynchronously with the emitter according to claim 4. This arrangementinvolves particularly low cost structurally and has the advantage thatthe measurement values of several measurement paths can be obtained fromonly one ray and only one detector element.

Particular operational advantages are achieved with the features ofclaim 7. Preferably, each of the two emitters supplies half themeasuring paths.

According to claim 8, several density profiles from each of the narrowfaces can be established simultaneously. This procedure is sparing oftime and leads to particularly reliable classification of the quality ofthe workpieces.

The features of claim 9 can be used in particular with stationaryworkpieces and with a traversing measuring device.

The aforesaid object of the invention is also achieved by the methodfeatures of claim 10, with the same advantages as for claim 1.

According to claim 11, one achieves a particularly rapid reaction in theevent of errors arising in the manufacturing process.

According to claim 12, the measurements for establishing the densityprofile can be performed without lengthening the cycle time with theworkpiece stationary. One can use for this for example the stoppage timeof the panel-like workpieces in the so-called cooling turner.

According to claim 13 the emitter is preferably moved at right-angles tothe direction of the radiation.

With the features of claim 14 one achieves a particularly rapid andaffirmatory qualitative analysis of the workpiece.

The features of claim 15 are particularly favourable from the point ofview of structural cost.

Further features and advantages of the invention will become apparentfrom the following description of a number of embodiments of theinvention which are given by way of example and with reference to thedrawings. In the drawings:

FIG. 1 is a schematic representation of a side view of a first measuringapparatus;

FIG. 2 is the view taken along the line II--II in FIG. 1;

FIG. 3 is a schematic illustration of a side view of two measuringdevices associated with a workpiece;

FIG. 4 is a schematic illustration of a density profile through thethickness of the workpiece;

FIG. 5 shows a workpiece with a plurality of stationary measuringdevices at several narrow faces;

FIG. 6 shows a workpiece with a measuring device movable along a narrowface;

FIG. 7 is a schematic illustration of another measuring apparatus whoseemitter is movable through the thickness of the workpiece;

FIG. 8 is a schematic side view of another measuring apparatus which ismovable in toto through the thickness of the workpiece; and

FIG. 9 is a schematic circuit diagram of an apparatus for establishing adensity profile.

FIG. 1 is a schematic illustration of an apparatus 5 for establishing adensity profile through the thickness 6 of a panel-like workpiece 7, forexample a particle board panel or fibre board panel. A measuring device8 comprises an emitter 9 and a linear array 10 of detector elements, ofwhich only detector elements 11 to 14 are shown in FIG. 1.

The emitter 9 harbours for example a radioactive isotope which emitsgamma radiation in the form of a parallel-sided radiation beam 15. Theheight 16 of the radiation beam 15 is indicated in FIG. 1.

In order to produce the radiation beam 15 the emitter 9 is provided witha narrow rectangular slit 17 (FIG. 2) having a thickness 18.

From the radiation beam 15, which is continuous in the direction of itsheight 16, four rays 1 to 4, for the purpose of the illustration, areselected and subsequently analysed in more detail. The rays 1 to 4 areparallel to one another and each penetrate into a narrow face 20 of theworkpiece 7 at an angle 19 which is less than 90° and more than 0°. Therays 1 to 4 thus pass through the workpiece 7 along paths 21 to 24 whoselengths decrease respectively in this sequence. At the ends of the paths21 to 24 the rays 1 to 4 leave the workpiece again and impinge on thedetector elements 11 to 14. The radiation beam 15, the paths 21 to 24and the linear array 10 with its detector elements, for example 11 to14, lie in a common measuring plane which is at least approximatelyperpendicular to the narrow face 20 of the panel. References to thenarrow face 20 of the panel are to be understood as meaning one of thetwo end faces or alternatively one of the two side faces of theworkpiece 7.

With particle board panels and fibre board panels one endeavours toachieve surface layers of comparatively high density and a central corelayer of lesser density between the surface layers. In FIG. 1 fourslices 25 to 28 are indicated representing the total thickness 6 andwhich in this sequence have respective densities p₁, p₂, p₃ and p₄.

The ray 1 travels in the slice 25 a distance d₁, thereafter in the slice26 a distance d₂, then in the slice 27 a distance d₃, and finally in theslice 28 a distance d₄. The sum of these distances d₁ to d₄ is equal tothe path length 21. The ray 2 traverses the distances d₁ +d₂ +d₃, inother words the path 22. The ray 3 traverses the distances d₁ +d₂, thusthe path 23. Finally, the ray 4 traverses only the distance d₁ which isequal to the path length 24.

The initial intensity I_(O) of all the rays 1 to 4 before their entryinto the narrow face 20 of the panel is the same. Because of thedifferent length paths 21 to 24 and the different densities which occuralong the paths 21 to 24, the rays 1 to 4 are attenuated to differentdegrees on their journeys through the workpiece 7, so that the detectorelements 11 to 14, in this sequence, detect increasing outputintensities I₁ to I₄ of the respective rays 1 to 4.

A decisive factor for the attenuation of each ray 1 to 4 in its passagethrough the workpiece 7 is Lambert's law

    I=I.sub.0.e.sup.-μpd

in which:

I₀ the initial input intensity of the ray [counts per second]

μ the absorption coefficient [cm² /g]

p the density [g/cm³ ]

d the path length travelled by the ray in the workpiece [cm]

I the output intensity of the ray after traversing the absorbingworkpiece [counts per second]

From this the output intensities I₁ to I₄ of the rays 1 to 4 result inthe following form:

    I.sub.1 =I.sub.0.e.sup.-μ(p.sbsp.1.sup.d.sbsp.1.sup.+p.sbsp.2.sup.d.sbsp.2.sup.+p.sbsp.3.sup.d.sbsp.3.sup.+p.sbsp.4.sup.d.sbsp.4.sup.)

    I.sub.2 =I.sub.0.e.sup.-μ(p.sbsp.2.sup.d.sbsp.2.sup.+p.sbsp.3.sup.d.sbsp.3.sup.+p.sbsp.4.sup.d.sbsp.4.sup.)

    I.sub.3 =I.sub.0.e.sup.-μ(p.sbsp.3.sup.d.sbsp.3.sup.+p.sbsp.4.sup.d.sbsp.4.sup.)

    I.sub.4 =I.sub.0.e.sup.-μ(p.sbsp.4.sup.d.sbsp.4.sup.).

With these output intensities I_(n), a density profile (see FIG. 4) isestablished for the narrow face 20. This arises basically from the factof there being a difference between two adjacent output intensities. Inpractice, the linear array 10 consists of very many more than just thefour detector elements 11 to 14 which are shown in FIG. 1, dependingupon the resolution that is wanted.

In FIG. 1, below the ray 4, there is indicated a ray 29 which travels aminimum path in the lower corner at the lower end of the narrow face 20of the workpiece 7. The detector element associated with the ray 29, butwhich is not shown in FIG. 1, consequently registers only a minimumattenuation of the initial intensity to give a datum output intensity ofthe ray 29. From this datum output intensity of the ray 29 is thensubtracted for example the output intensity I₄ of the ray 4. The outputintensity I₄ is smaller than the output intensity of the ray 29, fromwhich the density at the lower end of the narrow face 20 can becalculated. From the aforementioned difference one can also calculatethe density of the workpiece 7 at any point at which the ray 4 entersthe narrow face 20. By similar difference calculations one can calculatethe density values of all points at which the individual rays of theradiation beam 15 enter into the narrow face 20.

In all the Figures of the drawings the same or equivalent components areindicated by the respective same reference numerals.

According to FIG. 3, two emitters 9 and 30 are used which emitrespective radiation beams 15 and 31 each at the angle 19 to the panelnarrow face 20. The emitters 9, 30 however are arranged as mirror imagesone of the other, so that the radiation beam 15 only penetrates theslices 27, 28 and the radiation beam 31 only penetrates the slices 25,26. Besides the measuring device 8, in FIG. 3 there is also provided afurther measuring device 32 which comprises a further linear array 33 ofindividual detector elements.

A lowermost ray 34 of the radiation beam 31 travels in the slice 26 adistance d₂₆ and in the slice 25 a distance d₂₅. All the other rays ofthe radiation beam 31 travel shorter overall distances within theworkpiece 7. It is of particular advantage for the determination of thedensity profile in this way that for a given thickness 6 of theworkpiece 7 the maximum measurement distance d₂₆ +d₂₅ is shorter than isthe case in FIG. 1, where the single radiation beam 15 has to irradiatethe full depth of the narrow face 20 of the panel.

FIG. 4 shows a typical density profile 35 through the thickness 6 of theworkpiece, in this case a particle board panel. An average value 36 ofthe density is indicated by a broken horizontal line. The maxima 37 and38 of the density profile 35 lie, as desired, right towards the outside,in which region of the surfaces of the workpiece 7 particularly highdensity values are aimed for. The zones in FIG. 4 to the left of themaximum 37 and to the right of the maximum 38 are in the usual wayremoved later by sanding or calibrated grinding, so that the maxima 37,38 of the density are finally the actual values in the outer surfaces ofthe workpiece.

FIG. 4 shows also that one can have comparatively small density valuesin the core layer of the particle board panel between the two surfacelayers.

In the apparatus 5 shown in FIG. 5 two stationary measuring devices 8 asshown in FIG. 1 are set laterally spaced from one another at the narrowface 20 of the panel. At the adjoining narrow face 39 of the workpiece 7there is positioned a further stationary measuring device 8. Preferably,the workpiece 7 is held fast in one measuring position relative to thedifferent measuring devices 8. In the finishing process for theworkpieces 7 it is usually necessary for there to be periods when theworkpieces 7 are at a standstill, during which periods the measurementsfor establishing the density profile can be carried out withoutlengthening the total cycle time.

The apparatus 5 shown in FIG. 6 comprises only one measuring device 8 atthe narrow face 20. The measuring device 8 is displaceable along thenarrow face 20 in the directions of the double-headed arrow 40 on aguide rail 41 by means which are not shown. During this movement orwhile the measuring device 8 is stationary, the density profile atdifferent positions along the length of the narrow face 20 is taken. Inthe same way, measurement values for establishing density profiles couldbe obtained by a measuring device movable along the narrow face 39.

Both in FIG. 5 and also in FIG. 6 density profiles not only for thenarrow faces 20 and 39 but also for the oppositely disposed narrow facescan be determined.

In the case of the apparatus 5 shown in FIG. 7 the emitter 9 of themeasuring device 8 does not emit a radiation beam but only one ray 42which after impinging on the narrow face 20 traverses the full thickness6. For this, the emitter 9 is displaceable by means which are not shownalong a guide rail 44 in the directions of the double-headed arrow 43.The linear array 10 is stationary. Its individual detector elements arestruck by the ray 42 which is decreasingly attenuated in intensity.Movement of the emitter 9 in FIG. 7 is effected from the top downwards.

The basic structure of the apparatus 5 shown in FIG. 8 is similar tothat shown in FIG. 7. In FIG. 8 however, instead of the linear array 10comprising a plurality of detector elements as shown in FIG. 7, only oneindividual detector element 45 is provided which is fixed by means of aholding device 46 to the housing of the emitter 9. In this way thedetector element 45 follows all movements of the emitter 9 in thedirections of the double-headed arrow 43. The detector element 45registers in succession the different output intensities which resultduring the irradiation of the narrow face 20 by the ray 42, and fromwhich the corresponding different density values are obtained.

As shown in FIG. 9, each detector element 11 to 14 is connected by meansof a lead 47 to 50 with an evaluation circuit 51. Each detector element11 to 14 produces an electrical signal corresponding to the outputintensity of the attenuated radiation, and these signals are evaluatedin the evaluation circuit 51. The evaluation circuit 51 is connected bymeans of a lead 52 to a computer 53. Connected to the computer by leads54 to 56 are a viewing screen 57, a printer 58 and a storage device 59.The density profile 35 (FIG. 4) can thus be reproduced on the one handon the screen 57 and on the other hand can be printed out by the printer58. It can also be stored in the storage device 59 for subsequent otherpurposes.

I claim:
 1. An apparatus for establishing a density profile through thethickness of a panel-like workpiece of non-homogeneous material havingnarrow faces, comprising:a measuring device emitter for emittingradiation which passes through the workpiece whereby the radiation isattenuated by absorption in dependance upon the local density of theworkpiece, said emitter arranged to direct the radiation into a narrowface over the full thickness of the workpiece in a plurality ofmeasuring paths lying in one measuring plane in a direction which isinclined at an angle between 90° and 0° relative to said narrow face ofthe workpiece; a measuring device detector arranged to detect theradiation passing through the workpiece in each of said measuring paths,said detector capable of producing an electrical signal corresponding tothe output intensity of the radiation being detected; and means forestablishing the density profile by difference calculations on theoutput intensity of the radiation detected in each of said measuringpaths, said detector being electrically connected to said means forestablishing the density profile.
 2. An apparatus according to claim 1wherein said emitter is arranged so that the measuring paths areparallel to each other and the measuring plane is arranged approximatelyperpendicular to the narrow face.
 3. An apparatus according to claim 1wherein said emitter is adapted to direct the radiation in all saidmeasuring paths by a common radiation beam emitted by the emitter.
 4. Anapparatus according to claim 1 wherein said emitter includes means fordirecting in succession a ray of radiation to all said measuring paths.5. An apparatus according to claim 1 wherein said detector includes adetector element for each said measuring path, each said detectorelement connected electrically to said means for establishing thedensity profile.
 6. An apparatus according to claim 4 wherein saiddetector comprises only one detector element for detecting said ray andis connected electrically to said means for establishing the densityprofile.
 7. An apparatus according to claim 1 wherein said emittercomprises a first and second emitter, said first emitter arranged todirect radiation in a portion of said plurality of measuring paths whichare more remote from said first emitter, said second emitter arranged todirect radiation in a remainder of said measuring paths and arranged asa mirror image of said first emitter relative to said workpiece.
 8. Anapparatus according to claim 1 comprising a plurality of measuringdevices wherein each of said measuring devices includes at least onesaid emitter and at least one said detector, said plurality of measuringdevices being arranged spaced from one another and positioned to directradiation into the same said narrow face.
 9. An apparatus according toclaim 1 wherein a measuring device including said emitter and saiddetector is moveable to different positions in succession relative tosaid narrow face for determining the density profile.
 10. A method forestablishing a density profile through the thickness of a panel-likeworkpiece of non-homogeneous material, comprising the steps of:(a)directing radiation from an emitter into a narrow face of the workpiece,said radiation being directed in a direction inclined at an anglebetween 90° and 0° relative to a narrow face over the full thickness ofthe workpiece in a plurality of adjacent measuring paths lying in onemeasuring plane and adjacent to one another, (b) allowing the workpieceto be traversed by the radiation whereby the radiation is attenuatedduring the traverse of the workpiece, (c) detecting the attenuatedradiation traversing the workpiece in all said measuring paths with adetector, (d) producing from the detector electrical signalscorresponding to the output intensity of the attenuated radiation ineach of said measuring paths, and (e) using the electrical signalsobtained from step (d) in an analyzer to establish the density profile,wherein the density profile is established by difference calculations onthe output intensities of the attenuated radiation detected inrespective adjacent measuring paths.
 11. A method according to claim 10wherein the density profile is established "on line" during themanufacturing process of the workpiece without destruction.
 12. A methodaccording to claim 11 wherein the density profile is established whilethe workpiece is stationary.
 13. A method according to claim 10 whereinsaid emitter is moved transversely to the direction of the radiation inorder to establish the density profile.
 14. A method according to claim10 wherein the density profile is established substantiallysimultaneously for a plurality of positions on the narrow face of theworkpiece.
 15. A method according to claim 10 wherein a measuring devicecomprising said emitter and said detector is moved along said narrowface of the workpiece, and the density profile is established by themeasuring device at different positions on the narrow face insuccession.
 16. An apparatus according to claim 2 wherein said emitteris adapted to direct the radiation in all said measuring paths by acommon radiation beam emitted by the emitter.
 17. An apparatus accordingto claim 2 wherein said emitter includes means for directing insuccession a ray of radiation to all said measuring paths.
 18. Anapparatus according to claim 2 wherein a measuring device including saidemitter and said detector is moveable to different positions insuccession relative to said narrow face for determining the densityprofile.
 19. A method according to claim 11 wherein said emitter ismoved transversely to the direction of the radiation in order toestablish the density profile.
 20. A method according to claim 11wherein the density profile is established substantially simultaneouslyfor a plurality of positions on the narrow face of the workpiece.
 21. Amethod according to claim 11 wherein a measuring device comprising saidemitter and said detector is moved along said narrow face of theworkpiece, and the density profile is established by the measuringdevice at different positions on the narrow face in succession.
 22. Anapparatus for determining a density profile through the thickness of apanel, comprising:an emitter positioned to direct radiation into anarrow face of the panel over the full thickness of the panel in aplurality of adjacent measuring paths lying in one measuring plane in adirection which is inclined at an angle between 90° and 0° relative tosaid narrow face; a detector for producing an electrical signalcorresponding to the intensity of the radiation it detects, saiddetector positioned to detect the radiation passing through said panelof each of said measuring paths; and analyzer means for establishing thedensity profile by difference calculations on the intensity of theradiation detected in each of said measuring paths, said detector beingelectrically connected to said analyzer means.